In reply to CrushUnit:
It's a cracking lens, sharp and contrasty. It is well built and the IS works brilliantly. If you can live with the f4 aperture by all means, go for it. It has advantages over the non IS f2.8 such as IS (obviously), much lighter and smaller which comes into play more if you have a small camera body (my f2.8 wasn't great on a 400D, decent on 40D but brilliant on 1D because of balance), smaller filters and it is also sharper. The only thing I'd suggest though if you are looking at the f4 IS is to consider the 70-300L lens. Sharp as anything, longer zoom range but obviously a stop of light loss at the long end. The Tamron 70-200 f2.8 VC is also getting stellar reviews but is a bit more expensive than the canon f4.
If your primary lens is a 24-105 or 17-55 the 70-200 f4 seems like an ideal companion. If you are shooting sport or strobist stuff get the non IS 70-200 but for everything else IS is a god send with a focal length like that. f4 will give you very blurred backgrounds if you are a bit careful, especially portraits at 200mm